1. Field of the Invention
The present invention relates to a start control device of an internal combustion engine which performs starting by cranking in response to a start request.
2. Description of the Related Art
A reciprocating engine (internal combustion engine) to be installed in an automobile (vehicle) is started by cranking an engine when a starter is actuated in response to a start request issued by an operation on a start switch such as an ignition switch or a push switch.
Normally, in an engine, to reduce a load on starting, by setting a valve closing timing of an intake valve to a timing separated toward the top dead center apart from the bottom dead center of a compression stroke, an effective compression ratio is lowered and the load is reduced.
Recently, this valve closing timing of the intake valve is set by using a variable valve mechanism which changes the opening and closing timings of the intake valve.
Starting performance of an engine at an extremely low ambient temperature is demanded. Recently, additionally, starting performance in the case where, instead of a normal fuel such as gasoline, a fuel with different fuel properties such as an alcohol mixed fuel (mixed fuel) being hard to ignite is used has been demanded.
However, the alcohol mixed fuel is harder to vaporize than a normal fuel (gasoline). Therefore, with a related-art setting of the closing valve timing of the intake valve, an engine is hard to start. In particular, a fuel mixture ratio of the alcohol mixed fuel changes each time of fueling, so that the poor starting performance is also varied.
Therefore, starting performance of an engine adaptable to not only a normal fuel but also alcohol mixed fuels (0 to 100%) has been demanded.
Therefore, as disclosed in JP-A-2007-198308, a concentration sensor is provided inside a fuel tank of an automobile (vehicle) and a variable valve mechanism which changes a closing timing of an intake valve is adopted, and when starting an engine, the valve closing timing of the intake valve is changed (advanced) to a target valve closing timing on the bottom dead center side by the variable valve mechanism according to a fuel mixture ratio obtained from an alcohol concentration in the fuel in the tank detected from the concentration sensor. This is a technique of starting an engine by cranking upon making an environment with a higher internal cylinder temperature (by increasing an actual compression ratio of the cylinder), that is, an environment in which the fuel easily burns inside the cylinder by changing the valve closing timing of the intake valve.
According to this technique, the closing timing of the intake valve is advanced, the actual compression ratio of the cylinder increases, and the internal cylinder temperature rises, so that the fuel easily ignites.
However, the alcohol concentration in the fuel tank changes each time of fueling of the alcohol fuel. Further, in many cases, the fuel in the fuel line from the fuel tank to the engine, to be used for starting the engine in actuality has a different fuel mixture ratio from that at the time of fueling, and when starting the engine, the fuel mixture ratio of the alcohol mixed fuel when it is injected into the cylinder is unknown.
According to the technique described in JP-A-2007-198308, while this fuel mixture ratio is unknown, the valve closing timing of the intake valve is advanced to the valve closing timing set according to the alcohol concentration in the fuel tank, so that the engine starting performance inevitably becomes easily unreliable. Further, according to the technique described in JP-A-2007-198308, cranking is performed after the valve closing timing of the intake valve is changed, so that the load on starting is great, and the original measure for reducing the load is lost. In addition, according to the technique described in JP-A-2007-198308, a sensor for detecting a fuel mixture ratio of the alcohol mixed fuel, such as detecting the alcohol concentration, is needed separately, and this increases the cost burden in addition to the unreliability.